Elliptic curves with Galois-stable cyclic subgroups of order 4

Abstract

Infinitely many elliptic curves over $\mathbf{Q }$ have a Galois-stable cyclic subgroup of order 4. Such subgroups come in pairs, which intersect in their subgroups of order 2. Let $N_j(X)$ denote the number of elliptic curves over $\mathbf{Q }$ with at least j pairs of Galois-stable cyclic subgroups of order 4, and height at most X. In this article we show that $N_1(X) = c_{1,1}X^{1/3}+c_{1,2}X^{1/6}+O(X^{0.105})$. We also show, as $X\rightarrow \infty$, that $N_2(X)=c_{2,1}X^{1/6}+o(X^{1/12})$, the precise nature of the error term being related to the prime number theorem and the zeros of the Riemann zeta-function in the critical strip. Here, $c_{1,1}= 0.95740\ldots$, $c_{1,2}=- 0.87125\ldots$, and $c_{2,1}= 0.035515\ldots$ are calculable constants. Lastly, we show no elliptic curve over Q has more than 2 pairs of Galois-stable cyclic subgroups of order 4.